Page 181 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Calcium: Hypercalcemia and Hypocalcemia 171

alsosignificantlydecreasedwithtrilostanetherapy,butthere diagnosis of primary hypoparathyroidism in cats 444 and
was still a significant difference post-treatment between the 1 day to 25 weeks (mean, 3 weeks) before diagnosis in
dogs with hyperadrenocorticism and the control group. dogs. 87 Primary hypoparathyroidism and parathyroiditis
occur in dogs and cats of any age but more frequently
EFFECTS OF DRUGS in female dogs and male cats. In 735 dogs with primary
Drug administration may cause a decrease in iCa. A sig- hypoparathyroidism, 62% were female and 38% were
nificant decrease in iCa was observed in dogs male. 466 Mean age was 7.0 3.9 years, with 71% of
administered enrofloxacin at 5 mg/kg intramuscularly diagnoses occurring in purebred dogs. The highest odds
once daily for 14 days. 138 Mean iCa decreased to its nadir ratios for hypoparathyroidism correcting for breed popu-
on day 3, remained below normal at day 10, and larity occurred in the standard schnauzer, Scottish terrier,
normalized by day 14 despite continued administration miniature schnauzer, West Highland white terrier, and
of enrofloxacin. dachshund. Reduced risk was identified for the German
Theadministrationofmithramycinorbisphosphonates shepherd dog, shih tzu, and Labrador retriever. In
can cause mild hypocalcemia asa side effect in humans, but another study, 357 dogs were diagnosed with primary
symptomatic hypocalcemia is rare. 570 The potential for hypoparathyroidism over a 2-year period. 521 Mixed-
development of hypocalcemia exists in dogs following breed dogs accounted for 25% of the cases, with 13%
mithramycin administration because normal dogs and schnauzers, 7% Labrador retrievers, 5% dachshunds, 4%
those with malignancy-associated hypercalcemia undergo Yorkshire terriers, 4% poodles, 3% golden retrievers,
significant decreases in serum iCa and tCa. 486,487 Use of and 3% Scottish terriers without correction for breed pop-
mithramycin is reserved for emergency management ularity. There were 59 other dog breeds represented with
of hypercalcemia refractory to other treatments because an incidence of less than 3% each. In a study of 17 dogs,
of severe toxicity in some dogs. mixed-breed dogs, German Shepherd dogs, Saint
Phosphate enema administration can result in hypocal- Bernards, and terrier breeds were most commonly
cemia after rapid absorption of phosphate, hyperpho- affected. 503
sphatemia, and subsequent mass law interaction with Serum tCa concentration is usually less than 6.5 mg/
serum calcium. This is particularly a problem in cats dL (often 4.0 to 4.9 mg/dL) in dogs with primary hypo-
and small dogs in which death can occur. 18,281,512,574 parathyroidism. Dogs that have episodes of tetany or
Serum tCa decreased within 45 minutes of administration seizures often have serum tCa concentrations less than
of a hypertonic phosphate enema to cats and persisted for 6.0 mg/dL. Serum phosphorus concentration is greater
4 hours. 18 Mean serum phosphorus was more than than serum calcium concentration in nearly all affected
14 mg/dL within 15 minutes, and increases persisted dogs and cats, and most have hyperphosphatemia. Para-
for 4 hours. Serum tCa concentrations were negatively thyroid gland biopsy may confirm the diagnosis of lym-
correlated to serum phosphorus. Mild hypernatremia, phocytic parathyroiditis as the cause of primary
severe hyperphosphatemia (mean, 37.6 mg/dL), and hypoparathyroidism, but the parathyroid glands can be
hypocalcemia were noted in five cats. Phosphate enemas difficult or impossible to locate during surgical explora-
should not be used in small dogs, cats, or in debilitated tion because of atrophy and fibrosis. Parathyroid gland
patients of any size. biopsy is not recommended to confirm hypoparathyroid-
ism since the advent of validated PTH assays for use in the
Hypoparathyroidism dog and cat.
Hypoparathyroidism is an absolute or relative deficiency
of PTH secretion that can be permanent or transient. Diagnosis of Hypoparathyroidism
Hypocalcemia and clinical signs referable to low iCa con- Inappropriately low concentrations of PTH result in
centration are the hallmarks of advanced hypoparathy- hypocalcemia, hyperphosphatemia, and decreased
roidism. Hypoparathyroidism in dogs is most concentrations of 1,25-dihydroxyvitamin D (calcitriol).
commonly idiopathic, whereas surgical removal of or Hypocalcemia results from increased urinary loss of cal-
injury to the parathyroid gland during thyroidectomy cium (hypercalciuria), reduced bone resorption, and
to correct hyperthyroidism is the most common cause decreased intestinal absorption of calcium. Hyperphos-
in cats. phatemia results from decreased urinary loss of phospho-
Idiopathic chronic inflammation of parathyroid tissue rus (hypophosphaturia) that overrides the effects of
occurs sporadically in both dogs and cats but more com- decreased bone resorption and decreased intestinal
monly in dogs. It is presumed that parathyroiditis has an absorption of phosphorus (secondary to calcitriol deficit)
immune-mediated mechanism. Histopathologic study of on serum phosphorus concentration. PTH is a potent
affected parathyroid glands reveals inflammatory cell stimulator and phosphorus is a potent inhibitor of the
infiltration (lymphocytes, plasma cells, and neutrophils), 25-hydroxyvitamin D-1a-hydroxylase enzyme system in
fibrosis, and loss of secretory cells. 87,179,444,446,530 Clini- renal tubules. Consequently, the absence of PTH and
cal signs occurred 1 to 26 weeks (mean, 7 weeks) before the presence of hyperphosphatemia act together to